This invention relates to the regulation of variations in the sliver weight on cards, carding engines, draw frames and the like.
There is a need in the processing of textile fibres to be able to compensate for unavoidable irregularities in intermediate products by means of correcting devices. Such irregularities can be produced by properties of the material, properties of the machinery, by the personnel or by chance. In the final analysis, the better the intermediate products the better the finished products and the finished products can either be reduced to a better price or can produce less waste or scrap and thus save costs.
For economic reasons, stages of manufacture with a maximum yield, i.e. with a large amount of processed fibre per unit time are particularly suitable for the use of correcting devices. The card and the draw frame provide optimum properties for cotton spinning mills. Various measuring and regulating devices for card and draw frame slivers are also already known. A measuring device is usually provided at the machine outlet to continuously measure the fibre sliver delivered with respect to its cross-section and to produce a signal by means of which the amount of fibre supplied can be controlled by means of a regulating device in such a way that a weight of sliver which is as constant as possible per unit length (i.e. a sliver count which is as constant as possible) is delivered.
The apparatus which has been conventional up until now has a closed control circuit. However, owing to the relatively long path of the fibre between the regulating position and measuring position, such known apparatus has a dead time which makes it impossible in principle to regulate out errors which are shorter than the path of the fibre between the regulating position and measuring position. On the card with measurements on the take-off calenders and regulation of the speed of the feed roller, this path of the fibre corresponds to about 4 meters sliver length. In subsequent processing, the card slivers are, however, drafted by 100-fold to 1000-fold so that errors of at least 400 to 4000 meters length are still contained in the finished yarn in spite of regulating apparatus. Any significant reduction in these residual errors, therefore, denotes a technical progress toward improvement in the quality of the textile end product.
The draw frame which usually follows a card has until now also been subjected to similar restrictions with regard to regulation in the closed control circuit. Although in this case the distance between regulating position and measuring position is smaller, it is still sufficiently large for considerable dead times to exist, and thus residual errors remain in the sliver. Since virtually none of the subsequent processing operations allows regulating out, the residual errors also act, in the final analysis, as quality-impairing count variations in the finished product.
Some proposals for improvement are known, in particular for reducing the correction length on cards, carding engines and draw frames. However, these known proposals are burdened with considerable disadvantages. For example, the following measures have been considered on cards and found disadvantageous:
(a) various means of regulating the drafting mechanism, and the drawing of the drafting mechanism at the outlet of the card being modified in opposition to the variations in the cross-section of the sliver. Disadvantages of this measure are that it is mechanically complicated and expensive, particularly in the case of subsequent regulator modification;
(b) regulation of the drawing between the doffer and take-off calender is disadvantageous in that it involves considerable interventions into the card, has a small control region, is unflexible, and results in difficult servicing;
(c) measuring the weight of the fibre material introduced at the card inlet in the open control circuit. This approach is disadvantageous because the measuring member is complicated and imprecise owing to a poor opening with regard to short deviations, and has high sensitivity to differences in the opening of the flocks in the feed sliver.
In draw frames, one method of reducing the correction length which has been considered is the measurement of the entering fibre slivers and evaluation in an open control circuit. In the present state of the art, this measurement can be made virtually only with a mechanical measuring member or an isotopic measuring member. Disadvantages of this method are that an isotopic measuring member encounters pyschological resistance and demands additional safety measures. With a mechanical measuring member, the marked compression of the entering slivers is disadvantageous in regard to the subsequent drafting. In addition, an open control circuit demands very precise conformity between the characteristic of the measuring member and regulating member, which is virtually unobtainable for long and large deviations from the desired value of the feed sliver.